1. The Field of the Invention
The present invention relates generally to a method and an apparatus for measurement of communication quality in a mobile communication system using a CDMA cellular system. More particularly, the invention relates to methods and systems for detection of synchronization suitable for measuring communication quality using channels spread by relatively long known spreading code constantly transmitted by installed base stations.
2. The Relevant Technology
In CDMA cellular systems, it is required to detect a synchronization chip timing of a channel to measure, namely a reception chip timing in conventional measurement of communication quality (propagation characteristics). It should be noted that the reception chip timing is premised to match with a position of a path between transmitter and receiver in time effective for communication. Next, for setting a correlation detector called as finger for the reception chip timing for deriving a received signal vector in the reception chip timing from a correlation value.
FIG. 1 shows a process for deriving a desired signal power and an interference signal power. The signal vector set forth above is a sum of the desired signal and the interference signal. Therefore, a received signal vector obtained within a certain given period (normally about 3 to 5 symbols) is summed and averaged to take obtained vector as a desired signal vector. The desired signal power is derived as a power of the desired signal vector. The interference signal vector is derived from a variance of the received signal vector with taking the desired signal vector as reference. Here, the chip length to be used in correlation is referred to as one symbol.
Effective paths between transmitter and receiver presents in about three to five paths in urban area in general (here, chip rate is about 4 Mcps). Accordingly, in order to obtain the desired signal power, the interference signal power and SIR for all of the effective paths, corresponding number of fingers are required to be not effective. On the other hand, vector averaging is included during process of measurement, it is essential that the symbol to be used for averaging is “a symbol which can be regarded to have the same phase of the desired signal vector”. Normally, the symbol continuous in time is used. However, the condition is different when a transmit diversity is applied to the channel to be measured.
For example, when the channel to be measured is taken as a common pilot channel when the transmit diversity is applied, at first, the received signal vector after correlation detection is added and subtracted by preceding and following symbols to separate into received signal vector per each transmission antenna. Next, the desired signal power and the interference signal power are derived by performing the foregoing calculation per each received signal vector. By combining the desired signal power and the interference signal power obtained for each transmission antenna, the desired signal power and the interference signal power upon use of the transmit diversity is derived.
During this process, the received signal vector for each transmission antenna can be obtained only at every two symbol period. Accordingly, number of samples to be used in averaging, namely number of received signal vectors, becomes smaller in comparison with the case where the transmit diversity is not applied, to degrade measurement precision for each transmission antenna. When number of samples is set to be the same number as that when the transmit diversity is not applied, phase variation of the desired signal vector due to fading becomes not ignorable. Naturally, the measurement precision for each transmission antenna is lowered, the measurement precision after final combination is also lowered.
On the other hand, when a measurement system is installed on a mobile measurement vehicle for measuring communication quality while moving, the path position between transmitter and receiver is moved per each path according to movement of the mobile measurement vehicle. Accordingly, upon measuring, detection of reception chip timing has to be constantly performed and position of the finger has to be updated. When updating speed is low relative to movement of the path, the reception chip timing and path position can be offset. Therefore, obtained measured value cannot be correct.
As a method for detection of synchronization chip timing, namely a method that the mobile equipment detects of synchronization with the connecting base station at a preceding stage to perform measurement of communication quality, so-called 3-step cell search method is used in a wide band CDMA (hereinafter referred to as W-CDMA) cellular system. More particularly, so-called 3-step cell search method is a method performing detection of synchronization using three channels of PSCH (Primary Synchronization CHannel), SSCH (Secondary Synchronization CHannel), CPICH (Common Pilot CHannel) transmitted from the base station. A cause why three channels are used by the 3-step cell search method is that the code of the channel transmitted from the base station (scrambling code) is unknown.
However, in measurement system or the like, in which the base station as object for measurement (objective scrambling code), the 3-step cell search method cannot be effective synchronization detection method. For example, when CPICH transmitted from the base station is used as channel to be measured, detection of synchronization may be performed by using only scrambling code, in which CPICH of the base station as object for measurement is spread.
On the other hand, when it is not necessary to perform communication between the base station and the mobile equipment immediately after detection of synchronization and when the synchronization chip timing may be determined with a certain range, complete synchronization detection is not required. Furthermore, it is possible to perform measurement of a service area by installing pseudo base station in advance of actual installation of the base station. In such case, it is not efficient to transmit three channels corresponding PSCH, SSCH and CPICH by the pseudo base station for measurement.
By the conventional method, for measuring communication quality (propagation characteristics) at high precision, correlation detectors called as fingers are provided in number corresponding to number of the paths, and installation position of each finger (reception chip timing) has to be certainly matched with the position of the path. Also, it is not efficient for constraint condition for number of samples and sampling interval upon deriving the desired signal power, the interference signal power in each reception chip timing.
On the other hand, it is not efficient in that number of samples and sampling interval are different depending upon whether transmit diversity is applied or not.
Furthermore, in the conventional method, upon performing detection of synchronization of the channel to be measured, channel other than the channel to be measured is required to be inefficient.